As an example mission, at Blizzcon you can help Reno Jackson escape from a ruined temple. Each league member will be a part of the story and also a legendary card:

Brann Bronzebeard. League founder.

Elise Starseeker. Her card that shuffles a “map to the golden monkey” into your deck. The golden monkey replaces your entire hand and deck with legendary cards.

Reno Jackson. Fortune hunter of questionable ethics.

Sir Finley Murgleton. Gentleman murloc.

A new mechanic in the adventure is the “discover” mechanic. When you play a card with discover, it will present you with three random (but not entirely random depending on your deck?) cards, and you can pick the best card that fits your current situation.

I really like the new Hunter card, the “Exporer’s Hat”, which gives a minion +1/+1 and a deathrattle to return another hat to your hand. I play a mid-range hunter though, and this seems to push hunters further into lower-curve favoritism.